This disclosure relates generally to aircraft system health monitoring for overheat and fire detection systems. More particularly, this disclosure relates to aircraft system health monitoring using optical signals.
Overheat detection systems monitor various zones within an aircraft, such as bleed ducts where high temperature, high pressure air is bled from the compressor stage of an engine, or in the wheel well of an aircraft to sense overheated brakes and/or “hot” tires which indicate that the tire has a low air pressure or that the brakes are hot. Overheat detection can be used for any equipment on the aircraft that requires monitoring for overheat conditions, such as electric motors, compressors, etc. Bleed air is utilized for a variety of functions on the aircraft, such as engine and airframe anti-icing, internal cooling of the engine, cabin pressurization and environmental controls, pressurization of hydraulic reservoirs and seals, and others. The bleed air typically has a temperature between 100° F. and 1,100° F. depending on the distance that the bleed air has traveled from the engine. The high temperature and pressure of the bleed air means that the bleed air may damage the aircraft if a leak or rupture occurs in the bleed duct. As such, overheat detection systems have sensors that run the length of the bleed ducts, or along structures in the vicinity of the bleed ducts, to monitor for temperature changes that would indicate leaks or ruptures in the duct.
Prior art overheat detection systems typically utilize eutectic salt technology to sense an overheat event. The eutectic salt surrounds a central conductor and the eutectic salt is surrounded by an outer sheath. A monitoring signal is sent down the central conductor, and under normal operating conditions the eutectic salt operates as an insulator such that no conduction occurs between the central conductor and the outer sheath. When an overheat event occurs, however, a portion of the eutectic salt melts and a low-impedance path is formed between the central conductor and the outer sheath. The low-impedance path is sensed by an electronic controller, which generates an overheat alarm signal. When the overheat event has subsided, the eutectic salt re-solidifies and once again insulates the central conductor. Through the use of various salts to create a eutectic mixture, a specific melting point for the salt can be achieved; thereby allowing different eutectic salts to be used in different areas of the aircraft to provide overheat monitoring across a variety of temperatures. While the eutectic salt technology allows for overheat events to be detected, the eutectic salt technology merely provides a binary indication of whether an overheat event has or has not occurred.